The separation of gas streams, most notably air, into constituent components such as oxygen and nitrogen has been practiced for many years utilizing energy intensive processes for the recovery of various purities and volumes of the gas product. Chemical and/or physical adsorption of individual gas components, fractional cryogenic distillation of various gas components, and differential permeation through membrane media have all been practiced to recover components of gas streams and particularly to recover oxygen and/or nitrogen from air. These processes generally suffer from high utility costs or the lack of continuous or convenient regeneration or operation.
Various processes for separating gas streams, such as air, have been suggested in which at least some power is recovered from an effluent stream to provide a portion of the power requirements of the process itself.
For example, U.S. Pat. No. 4,132,766 describes a chemical air separation process wherein air, at elevated temperature and pressure, is separated into oxygen and nitrogen-rich waste streams by preferential chemical binding of oxygen from the air. The nitrogen-rich stream is then reduced in pressure through an expander before being vented. The expander drives the compressor to recover some power for the process.
To provide additional power for the operation of compression equipment in a gas separation process, U.S. Pat. No. 4,340,578 suggests that in a chemical air separation plant, the waste nitrogen stream, still containing residual oxygen, is combusted with a fuel. The hot effluent is then expanded in several stages through turbines and power is recovered.
Alternately, U.S. Pat. No. 4,560,394 discloses that air may be compressed in a compressor, reduced in temperature by heat exchange against process streams or external cooling means and then separated into oxygen and a nitrogen-rich effluent stream by passage over a semipermeable membrane. While some power is recovered by pressure reduction of the nitrogen-rich stream, no fuel is combusted and no net power is produced.
Power generation can be achieved using a cryogenic air separation process as described in U.S. Pat. No. 4,224,045. In that process, air is compressed and then cooled to its liquefaction temperature before being distilled through a fractionation column. The waste stream from the column is rewarmed, recompressed, and then combusted with fuel and by-pass air. The combusted effluent is expanded through a turbine to recover power for the process.
U.S. Pat. No. 4,545,787 teaches a method for the generation of net power and the recovery of by-product oxygen-rich gas at low power requirements. Air is compressed to an elevated temperature and pressure. At least a portion of the air is combusted and a portion of the oxygen is removed from the air or combustion effluent through a membrane or adsorbent before the oxygen-lean combustion effluent is expanded through a turbine to recover power for the process and net power. While this system is attractive, it is not always possible to include such an oxygen extraction device without substantial modification of the gas turbine construction and degradation in power generation performance. This is particularly true where the retrofitting of an existing simple gas turbine having no external combuster is required.